Heat the oat milk over medium-low heat until lukewarm (do not boil). Remove from heat, add the margarine, and stir until melted. Set aside.
In a small bowl, stir the active dry yeast into lukewarm water and let it rest until foamy (about 5 minutes).
In a stand mixer with a dough hook, combine the yeast mixture, milk, margarine, egg (or flax egg), sugar, salt, and cinnamon.
Add the flour and mix on low speed until a dough forms. Knead for about 4 minutes until the dough is smooth and pulls away from the sides of the bowl. The dough should not be sticky.
Transfer the dough to a lightly oiled bowl, cover with plastic wrap, and let it rise for 1 hour.
Shape the Dough
Once the dough has risen, roll it out on a lightly floured surface to about 3/8 inch thick (1 cm).
Use a heart-shaped cookie cutter to cut out shapes and place them on a floured baking sheet. Cover with a damp cloth and let them rise for another 35 minutes.
Deep-Fry the Berliners
Heat vegetable oil to 320°F (160°C) in a deep fryer or large pot. Fry a few Berliners at a time for about 3 minutes, turning them halfway through, until golden brown on both sides.
Remove from the oil and drain on paper towels. Let them cool completely.
Fill and Garnish
Fill a piping bag with raspberry jam. Poke a small hole in the side of each Berliner and pipe in the jam.
Once filled, dust with powdered sugar and serve immediately.